In certain applications, it is desirable or necessary to employ a multiple-frequency antenna having the following features: relatively broad bandwidth (about 10% or more); significant isolation between frequencies; ability to transmit/receive copolarized radiation; reliable; small size and low profile; and, easily produced at low cost.
One application in which the foregoing antenna characteristics may be desirable is in a two-way communication system which can transmit and receive signals simultaneously on separate frequencies. Broad bandwidth and isolation between the transmitting and receiving bands are important capabilities. Small size and low profile are particularly advantageous in mobile applications, including airborne radar arrays.
Microstrip antennas have been used in the foregoing applications and are known to be reliable and easily produced at a low cost. They are also small and have low profiles. A microstrip antenna generally includes a dielectric substrate having an electrically conductive reference surface disposed on one side and an electrically conductive radiating element disposed on the opposite side. The radiating element can be fed directly, such as with a co-axial connector or microstrip transmission line, or can be capacitively coupled to a feed. Bandwidths in excess of 10% can be achieved and individual microstrip antennas can be interconnected to form an array. Additionally, the small size and low profile of microstrip antennas enable them to be used where a conformal structure is required.
One known configuration of a multiple-frequency microstrip antenna comprises separate, adjacent, coplanar radiating elements disposed on a surface of a dielectric substrate (with a reference surface disposed on the opposite surface of the substrate). Feed locations on the radiating elements are selected for impedance matching and copolarized radiation can be accommodated; however, radiation from two adjacent radiating elements will not share a common phase center, making the layout of elements in an array more difficult to design. Furthermore, the use of such adjacent, coplanar elements is an inefficient use of space, a distinct deficiency in applications where space is at a premium. In order to meet broad bandwidth and out-of-band rejection requirements, the dielectric substrate must be relatively thick which can increase undesirable element-to-element coupling in an array. And, it will be appreciated that because the radiating elements share a single dielectric substrate having a single thickness, antenna performance cannot be optimized for each separate band.
In another known arrangement,.a single, dual-polarized radiating element is dimensioned to resonate at two frequencies in two orthogonal modes of excitation. However, such an arrangement suffers from gain isolation problems when, for example, polarized waves are received that are not aligned with a principal plane of the antenna. Clearly, copolarized radiation cannot be accommodated. Nor is it possible to optimize the Q-factor for each resonant frequency since the Q-factor is determined by the nonresonant dimension of a radiating element and by the substrate thickness. In the single element, dual-polarized configuration, the non-resonant dimension at one frequency is the resonant dimension at the other frequency. Thus, both the length and the width of the radiating element are determined by the desired resonant frequencies and it becomes difficult to adjust them to improve the Q-factor. And, because the antenna comprises a single radiating element on a single substrate, the substrate thickness cannot be optimized for both resonant frequencies. Consequently, radiation at the higher frequency will have a lower Q-factor and a broader response curve with roll-off characteristics which are undesirable in applications requiring good isolation between the operating bands.
Stacked microstrip antennas have also been used, comprising two or more radiating elements disposed above and parallel to a reference surface, separated from each other and the reference surface by dielectric layers. In some such antennas, a single feed is connected to one of the radiating elements and the one or more other radiating elements are electromagnetically coupled to the directly fed element. Alternatively, each radiating element can be separately and directly fed. It can be appreciated, however, that undesirable coupling can occur between radiating elements and between the feed elements, coupling which increases when the thicknesses of the dielectric layers are increased to obtain broader bandwidth. Such coupling is particularly pronounced when the radiation to/from the elements is copolarized. Furthermore, the roll-off characteristics may not permit the antenna to be used in a simultaneous, multi-frequency application.